In most missile guidance systems, there is a requirement for considerable dynamic range. This is due to the fact that the target signal strength is a function of the inverse square of the distance separating the missile and target. The usual limitation on linear processing of this signal is the biasing voltages of the processing circuits. The required compression is usually gained through the use of logarithmic amplifiers which can have a dynamic range of 100 dB. These amplifiers can be quite expensive, especially if they must be of the video type. Also, in certain low-noise applications, log amplifiers can be major contributors to the processing circuit noise. They require high quality precision components and can be rather difficult to temperature compensate. These log amplifiers, for the most part, do not readily lend themselves to implementation in the monolithic technology.
Fast rise-time logarithmic video amplifiers use a summation approach to approximate the logarithmic function. As such, they are really pseudo-logarithmic amplifiers. They are generally implemented by a multi-stage-parallel-summation technique. Each stage of a generalized logarithmic amplifier consists of a linear amplifier, an attenuator, and a logarithmic amplifier that has logarithmic characteristics over a limited span of the total dynamic range. The linear amplifiers and attenuators are selected to phase in each logarithmic stage sequentially as a function of input intensity. The stages generally provide from 15 to 20 dB dynamic range each.